The present invention relates to the manufacture of optical waveguide fiber preforms.
Optical waveguide fibers have been greatly improved during the last decade. Fibers exhibiting very low losses are generally formed by chemical vapor deposition (CVD) techniques which result in the formation of extremely pure materials. In accordance with these techniques, optical waveguide preforms can be formed by depositing glass layers on the outside surface of a temporary mandrel, or on the inside surface of a tube which later forms at least a portion of the cladding material, or by some combination of these techniques.
In accordance with one embodiment of the CVD technique, often referred to as the inside vapor deposition (IVD) process, the reactant vapors flow through a hollow, cylindrical substrate. The substrate and the contained vapor mixture are heated by a source that moves relative to the substrate in a longitudinal direction, whereby a moving hot zone is established within the substrate tube. A suspension of particulate material which is often called soot is produced within the hot zone. The soot travels downstream where at least a portion thereof comes to rest on the inner surface of the substrate where it is fused to form a continuous glassy deposit. Suitable layers are deposited to serve as the cladding, barrier layer and/or core material of the resultant optical waveguide fiber. The temperature of the glass tube is then increased to cause the tube to collapse. The resultant draw blank is then drawn in accordance with well known techniques to form an optical fiber having the desired diameter.
In another embodiment of the CVD process the vapor of reactant compounds is introduced into a suitable heat source such as a flame where it reacts to form a soot stream which is directed toward a mandrel. This so-called outside vapor deposition (OVD) method of forming coatings of glass soot is described in greater detail in U.S. Pat. Nos. Re 28,029; 3,823,995; 3,884,550; 3,957,474 and 4,135,901. After a plurality of coatings are formed on the mandrel, the mandrel is generally removed and the resultant tubular preform is gradually inserted into a consolidation furnace, the temperature of which is sufficiently high to fuse the particles of glass soot and thereby consolidate the soot preform into a dense glass body in which no particle boundaries exist. In one embodiment of the OVD process, which is described in U.S. Pat. No. 3,957,474, the starting rod forms the core of the resultant fiber. The deposited cladding soot is consolidated on the surface of the core rod. The resultant consolidated blank is drawn into an optical waveguide fiber. A modification of the OVD process referred to as axial vapor deposition is taught in U.S. Pat. Nos. 3,966,446, 4,017,288, 4,135,901, 4,224,046, and 4,231,774.
A hybrid technique whereby a core is formed by axial vapor phase oxidatin and a cladding layer is simultaneously deposited on the core by radially inwardly directed glass soot streams is taught in U.S. Pat. Nos. 3,957,474 and 4,062,665. As the core is formed, it is withdrawn from the burners or nozzles which formed it. The cladding is deposited by stationary burners or nozzles.
An important and probably limiting factor in determining the deposition rate in the aforementioned CVD processes is related to the temperature of the gas stream in which the soot particles are entrained. See the publication, P. G. Simpkins et al., "Thermophoresis: The Mass Transfer Mechanism in Modified Chemical Vapor Deposition", Journal of Applied Physics, Vol. 50, No. 9, September, 1979, pp. 5676-5681. Thermophoretic force drives the soot particles from the hotter parts of the gas stream toward the cooler parts. Because the preform surface is usually cooler than the surrounding gas stream, the action of thermophoresis tends to drive the soot particles toward the preform surface. When a surface is nearly as hot as the surrounding gas stream, the temperature gradient is low. Thus, the thermophoretic effect is minimal, and the deposition rate is low. However, when the surface temperature of the preform is low relative to that of the gas stream, the thermophoretic effect due to the large thermal gradient results in a relatively high deposition rate. Various techniques have been developed to increase deposition efficiency by increasing the thermal gradient between the gas stream and the substrate.
An embodiment which depends on an enhanced thermophoretic drive field produced by water-cooling the substrate tube is taught in U.S. Pat. No. 4,302,230. Enhancement of the thermophoritic field by disposing heaters within the substrate tube is taught in U.S. Pat. No. 4,263,032 and in U.S. patent application Ser. Nos. 161,011 and 161,012 now U.S. Pat. Nos. 4,328,017 and 4,328,018. U.S. Pat. No. 4,310,340 teaches the insertion of a flame-emitting tube into the substrate tube to enhance the thermophoresis effect. European patent application No. EP 0,038,982 A2 published Nov. 4, 1981 (MacChesney et al.) and equivalent to U.S. Pat. No. 4,331,462 teaches the use of an r.f. generated fireball to increase the thermal gradient within the substrate tube. Various of the above mentioned embodiments are disadvantageous in that they require the insertion of a heating apparatus into the substrate tube. The use of a plasma in the substrate tube is disadvantageous since the extremely high plasma temperatures can cause revolatilization of some of the soot particles, thereby causing the conversion of some of the GeO.sub.2 to GeO and oxygen.
U.S. patent application Ser. No. 270,235 now U.S. Pat. No. 4,378,985 teaches a method of improving the efficiency of the OVD process. Means such as a burner directs a stream of glass particulate material toward a lateral surface of a cylindrical core member to build up a coating thereon. The core member is provided with rotational movement and is provided with longitudinal movement in one direction with respect to the burner. Additionally, there is provided an oscillating movement of the burner with respect to a portion of the length of the core member. The oscillating motion of the burner relative to the core material permits the soot preform to cool down between successive burner passes, thus increasing deposition rate due to enhanced thermophoresis.